SQL Models Qt provides C++ classes that support SQL data models. These classes work transparently on the underlying SQL data, reducing the need to run SQL queries for basic SQL operations such as create, insert, or update. For more details about these classes, see Using the SQL Model Classes. Although the C++ classes provide complete feature sets to operate on SQL data, they do not provide data access to QML. So you must implement a C++ custom data model as a subclass of one of these classes, and expose it to QML either as a type or context property. Read-only Data Model The custom model must reimplement the following methods to enable read-only access to the data from QML: roleNames() to expose the role names to the QML frontend. For example, the following version returns the selected table’s field names as role names:
QHash<int, QByteArray> SqlQueryModel::roleNames() const { QHash<int, QByteArray> roles; // record() returns an empty QSqlRecord for (int i = 0; i < this->record().count(); i ++) { roles.insert(Qt::UserRole + i + 1, record().fieldName(i).toUtf8()); } return roles; }
data() to expose SQL data to the QML frontend. For example, the following implementation returns data for the given model index:
QVariant SqlQueryModel::data(const QModelIndex &index, int role) const { QVariant value;
if (index.isValid()) { if (role < Qt::UserRole) { value = QSqlQueryModel::data(index, role); } else { int columnIdx = role - Qt::UserRole - 1; QModelIndex modelIndex = this->index(index.row(), columnIdx); value = QSqlQueryModel::data(modelIndex, Qt::DisplayRole); } } return value;}
The QSqlQueryModel class is good enough to implement a custom read-only model that represents data in an SQL database. The chat tutorial example demonstrates this very well by implementing a custom model to fetch the contact details from an SQLite database. Editable Data Model Besides the roleNames() and data(), the editable models must reimplement the setData method to save changes to existing SQL data. The following version of the method checks if the given model index is valid and the role is equal to Qt::EditRole, before calling the parent class version:
bool SqlEditableModel::setData(const QModelIndex &item, const QVariant &value, int role) { if (item.isValid() && role == Qt::EditRole) { QSqlTableModel::setData(item, value,role); emit dataChanged(item, item); return true; } return false;
}
Note: It is important to emit the dataChanged() signal after saving the changes. Unlike the C++ item views such as QListView or QTableView, the setData() method must be explicitly invoked from QML whenever appropriate. For example, on the editingFinished() or accepted() signal of TextField. Depending on the EditStrategy used by the model, the changes are either queued for submission later or submitted immediately. You can also insert new data into the model by calling QSqlTableModel::insertRecord(). In the following example snippet, a QSqlRecord is populated with book details and appended to the model:
… QSqlRecord newRecord = record(); newRecord.setValue(“author”, “John Grisham”); newRecord.setValue(“booktitle”, “The Litigators”); insertRecord(rowCount(), newRecord); …
Exposing C++ Data Models to QML The above examples use QQmlContext::setContextProperty() to set model values directly in QML components. An alternative to this is to register the C++ model class as a QML type (either directly from a C++ entry-point, or within the initialization function of a QML C++ plugin, as shown below). This would allow the model classes to be created directly as types within QML:
C++
class MyModelPlugin : public QQmlExtensionPlugin { Q_OBJECT Q_PLUGIN_METADATA(IID “org.qt-project.QmlExtension.MyModel” FILE “mymodel.json”) public: void registerTypes(const char *uri) { qmlRegisterType(uri, 1, 0, “MyModel”); } }
QML
MyModel { id: myModel ListElement { someProperty: “some value” } }
ListView { width: 200; height: 250 model: myModel delegate: Text { text: someProperty } }
See Writing QML Extensions with C++ for details on writing QML C++ plugins.
